1. Field of the Invention
The present invention generally relates to electrical conductors and, more particularly, to coaxial cables for conducting relatively high frequency signals.
2. Description of the Related Art
There are known in the art certain connectors for coaxial cables which are commonly referred to as xe2x80x9cDC Blocksxe2x80x9d. These connectors are constructed to be located at an end of the cable assembly, which significantly restricts their utility and bars their use from applications to which the present invention is readily adapted. DC blocks are commonly used to eliminate ground loops, and to isolate sensitive electronics from adverse electromagnetic interference. Such DC Block connectors as are known are incapable of providing thermal or electrical separation in a hostile environment and they are not hermetic, as are embodiments of the present invention.
Certain patents of which the inventors are aware disclose gas-filled insulated casings for high voltage conductors which may superficially appear similar to embodiments of the present invention. Examples are found in U.S. Pat. No. 3,778,526 of Floessel, U.S. Pat. No. 4,011,118 of Geominy, U.S. Pat. No. 4,487,660 of Netzel et al. and U.S. Pat. No. 4,667,061 of Ishikawa et al. An air-dielectric coaxial cable with hollow spacer element is the subject of U.S. Pat. No. 5,742,002 of Arredondo et al. None of these disclosures is particularly relevant to the present invention for the reason that none of them shows a physical interruption in the outer conductor or sheath of the cable.
A data cable is disclosed in U.S. Pat. No. 5,990,419 of Bogese, II which comprises a single conductor cable with specially configured insulation; it is not a coaxial cable.
In brief, one particular arrangement in accordance with the present invention comprises a stub which is fabricated with a sleeve formed of two conductors that slide snugly onto the associated coaxial cable,in the complete assembly. The sleeve is in two parts with a dielectric insulation between them. One of the sleeves has an overlapping section of larger diameter and the dielectric insulation extends within this section between the two sleeve portions. The larger diameter section is necked down at the butt end of the stub to match the outer diameter of the coaxial cable and, at this point, the dielectric insulation extends into the space between the two sleeve portions at the surface of the coaxial cable. The sheath and dielectric insulation of the coaxial cable are cut and removed at the point where the space between the two sleeves of the stub is positioned. This results in a blockage of DC (direct current) and low frequency signals as well as thermal energy.
The electrical length of the stub is chosen such that it is equal to a quarter wavelength at the chosen frequency of operation. To achieve this condition, a series stub with an input impedance of zero ohms is used. Thus the stub terminates in an open circuit, thereby providing the physical separation desired. The stub has an impedance of infinity at the open end, which transforms to zero ohms at the junction with the coaxial cable. Therefore, at the operating frequency, the stub is transparent to the signal flowing in the cable assembly. However DC and lower frequencies of electrical signals are blocked.
In a preferred embodiment of the invention, in which the cable assembly has a chosen operating frequency of 4 GHz, the stub has a dielectric insulation of commercially available 7070 glass. Other insulation materials may be used to meet special requirements for thermal energy flow and power handling.
The most important variable in the structure of the invention is the length of the series stub. Once the dielectric material is chosen and the frequency of operation is determined, the length of the stub is found by the following equation:
L=0.075/(f(∈r)0.5)
where f is frequency in GHz, ∈r is the dielectric constant of the insulation used in the stub, and L is the stub length in meters.
Once the stub length is determined, the respective internal and external sleeve conductors can be fabricated. The external conductor is preferably made about 10% longer than the other conductor to allow for later adjustment. The space between the conductors is filled with the selected dielectric insulation. The shell is then fired to allow the dielectric to fill any gaps or voids and bond with the conductors to form a hermetic seal.
Next a section of the coaxial cable assembly is prepared by stripping a length of 1.27 mm from the outer conductor (shell) at the location where the;discontinuity is needed. The dielectric in that section may also be removed, although the center conductor is maintained intact.
After preparation of the chosen section of the coaxial cable as described, the stub is then slid onto the cable up to the stripped section. The stub is positioned so that the discontinuity of the coaxial sheath is located under the dielectric opening in the stub. The stub can then be welded to the outer jacket of the coaxial cable.
The length L in the formula above is measured from the midpoint of the gap or discontinuity in the coaxial cable. The thickness of the dielectric in the stub equals the length of this gap; the space between the two sleeve portions of the stub corresponds to the gap in the cable sheath. The material of the gap is not critical; it may be air or some other dielectric, depending upon the makeup of the ambient atmosphere in which the components are assembled. Alternatively, the gap may contain the insulation material of the cable if the material is not removed during removal of the portion of the sheath at the gap. The stub is now welded to the outer jacket of the coaxial cable. Connectors can be welded at both ends of the cable to complete the cable assembly.
The shell portion of the stub at the open end extends beyond the point of ideal length for the stub. This is to permit later adjustment after the stub is in proper position on the coaxial cable. At this point, the voltage standing wave ratio of the electrical signal as it travels through the cable assembly is measured with a network analyzer and stub length is adjusted as needed. Usually the outer conductor is longer than necessary and the stub can be shortened until the best voltage standing wave ratio at the desired operating frequency is achieved.